Blue Earth - The Story of Earth - page 52

Blue Earth
The Formation of the Oceans
Earth's Age: 100 to 200 million years
Earth's infancy, its first half-billion years or so, is shrouded in mystery. Rocks and min-
erals provide tangible evidence formost ofourplanet'sstoried past, but few rocks orminer-
als survive from that most ancient Hadean time. As a consequence, any narrative of Earth's
initial cooling and the subsequent watering of its black surface must be based on specula-
tions informed by experiments, models, and calculations. Even so, some uncertainties will
always remain.
That's not a bad thing. What makes each day at the lab new and exciting is the richness
of “what we know we don't know” and the possibility each day that we will discover some
small clue that brings us closer to truth. Even more tantalizing is the prospect of discover-
ing aspects of the natural world that “we didn't know we didn't know”—discoveries that
increase the breadth of mystery. * These new ways of asking questions—“How did miner-
als evolve?” for example, rather than simply “What are their chemical and physical proper-
ties?”—pave the way for breakthroughs.
It's important to take an inventory of what we don't know. All the evidence suggests that
the Moon formed by an epic impact, yet we can't be sure exactly when the collision oc-
curred, nor what were the nuances of Theia's final trajectory. Following that colossal colli-
sion,wecanimagineanincandescenttorrentialrainofsilicatesontoEarth'storturedmagma
ocean, but the duration and rate of cooling of such a superheated world are poorly con-
strained and will remain topics of much debate for decades to come. The proximity and re-
cession rate of the newly formed Moon, though critical to understanding the dynamics and
evolution of early Earth, are equally uncertain. Likewise, no one knows when the oceans
firstformed,norexactlywhattheylookedlike.Butformtheydid,andthefollowingstoryis
based on the best evidence available and so is as good as it gets for the time being.
The black Earth could not remain black for long. Global-scale volcanism spewed hot ni-
trogen, carbon dioxide, noxious sulfur compounds, and water vapor into the thickening at-
mosphere at rates of billions of tons per day. Those volatile elements and compounds—the
very same molecules that formed the varied ices of the former nebula, the very same atoms
thatyouarenowbreathingandthatmakeuptheintricatetissuesofyourbody—playedmany
roles in the rapidly evolving Earth. When hot water mixed with rock magmas, it lowered